Creates snapshot properly by SIGUSR1

https://bugs.launchpad.net/tarantool/+bug/1162797

by SIGUSR1 tarantool
- didn't touch box.info.snapshot_pid
- didn't waitpid for snapshot process

Conflicts:

	include/fiber.h
https://bugs.launchpad.net/tarantool/+bug/1162797 review

Remove added logging, a message with 'saving snapshot %filename'
is already printed to the log by the child. It contains
snapshot pid. Snapshot fid is not as useful once the bug
is fixed.
Merge branch 'stable'

Conflicts:
	include/fiber.h